The invention relates generally to spectroscopy systems and, more particularly, to a method and apparatus for improved signal-to-noise ratio in Raman signal detection for micro electromechanical system (MEMS) based spectrometer devices.
Spectroscopy generally refers to the process of measuring energy or intensity as a function of wavelength in a beam of light or radiation. More specifically, spectroscopy uses the absorption, emission, or scattering of electromagnetic radiation by atoms, molecules or ions to qualitatively and quantitatively study physical properties and processes of matter. Raman spectroscopy relies on the inelastic scattering of intense, monochromatic light, typically from a laser source operating in the visible, near infrared, or ultraviolet range. Photons of the monochromatic source excite molecules in the sample upon inelastic interaction, resulting in the energy of the laser photons being shifted up or down. The shift in energy yields information about the molecular vibration modes in the system/sample.
However, Raman scattering is a comparatively weak effect in comparison to Rayleigh (elastic) scattering in which energy is not exchanged. Depending on the particular molecular composition of a sample, only about one scattered photon in 106 to about 108 tends to be Raman shifted. Because Raman scattering is such a comparatively weak phenomenon, an instrument used to analyze the Raman signal should be able to substantially reject Rayleigh scattering, have a high signal to noise ratio, and have high immunity to ambient light. Otherwise, a Raman shift may not be measurable.
A challenge in implementing Raman spectroscopy is separating the weak inelastically scattered light from the intense Rayleigh-scattered laser light. Conventional Raman spectrometers typically use reflective or absorptive filters, as well as holographic diffraction gratings and multiple dispersion stages, to achieve a high degree of laser rejection. A photon-counting photomultiplier tube (PMT) or a charge coupled device (CCD) camera is typically used to detect the Raman scattered light.
Concurrently, however, there is a growing need for miniaturization of instruments for biological, chemical and gas sensing in applications that vary from medical to pharmaceutical to industrial to security. This is creating a paradigm shift in experimentation and measurement, where the trend is to bring the instrument/lab to the sample rather than bringing the sample back to the lab for analysis. Unfortunately, conventional methods of Raman detection that also provide sufficient signal-to-noise ratio (e.g., PMTs, CCD cameras) are generally not compatible with MEMS scaled devices, due to the bulk associated therewith. Accordingly, it is desirable to be able to detect Raman scattering with a higher signal-to-noise ratio given the constraints of smaller, on-chip “in the field” spectrometer devices.